I'd really like to read DiCenso's and Frip's explanation of what keeping time in the mind is as a practical matter. I'm not exactly sure what it means. I don't think I could keep good time without some physical reference, even if it's just nodding my head or my breathing rate. If I try to think about keeping time only in my mind, I still find myself thinking of some physical motion as a reference, even if I'm just singing the beat in my mind.
IMO, it's internalizing the time. In the same way you can internally "feel" 4, 8 and 16 bars or say how you feel 16ths, or 8th notes, etc
You don't have to think about it in any conscious manor. It's ingrained into the mind in the same way technique becomes ingrained in our muscle memory.
At a particular point a drummer no long consciously thinks of what his hands are doing consciously. The player crosses the into the realm of just playing, and letting his hands unconsciously translate the ideas coming from the mind. This I think is basically the same concept of internalizing 'time'.
Someone posted an article on the old forums that a guy talked about learning aurally VS physically and the internalization that comes from each, being different.
I believe if you "grew up" as a drummer marking time in a particular way, such as quarters on the hats with the foot, then the muscle memory and just overall learning wise, it becomes a dependent factor. Almost in a sense the body has learned to mark time physically, instead of internally. Perhaps making it that much harder to "re-wire" things so that you can develop and trust that internal time.
That video was SICK too, I really dig on Dave's approach with his book as well. Always great to get a fresh perspective on things.